Mechanical Seal – Practical Information Filed under: General Maintenance — K P Shah @ 1:43 pm
Modern process equipment with rotating shafts (such as pumps and compressors) are equipped with compression packing or mechanical seal to minimize emission of the process fluid into the atmosphere. Compression packing works on the principle of controlled leakage whereas mechanical seal tend to seal with no visible leakage.
The initial cost of a mechanical seal is high as compared to compression packing. However the power consumed, maintenance and downtime spent in renewing or tightening the compression packing overweigh the initial cost of a mechanical seal, which works unattended for a long time. Because of the absence of visible leakage, environment is clean and hazard free when mechanical seals are used. In this blog, information is given on working of a mechanical seal, types of mechanical seals, methods of environment control, equipment parameters, installation instructions, start-up procedure and check list for identifying causes of seal leakage.
Working of a mechanical seal
Basic Mechanical Seal
A basic mechanical seal is a simple device. It has two flat faces running against each other. The rotating face is secured to the pump shaft while the stationary face is held in the gland. This is the first and most important of the four possible leak paths (Primary Seal). This leakage path is sealed by providing absolutely flat mating surfaces perpendicular to rotating shaft centre line where they come in contact and maintaining healthy lubrication film between the two mating faces. Since both the surfaces are continuously moving with respect to each other, there is heat generation
which keeps on evaporating the liquid film and new liquid film is formed. These vapours keep on escaping to the atmosphere. Thus mechanical seal is not a zero leakage seal. There is always invisible leakage in vapour form between the faces.
The others three paths are: Between the Rotating Face and the Shaft (Secondary Seal), Between the Stationary Face and the Gland, and Between the Gland and the Stuffing Box.
Leakage at secondary seal is arrested by a dynamic O-Ring, sliding wedge or a bellow (elastomeric, PTFE or metallic). Metallic bellows are used for high temperature application.
The last two are jointly referred to as the “T ertiary Seal”, and both are fairly simple seals as there is no relative motion between the two parts involved. These leakage paths are sealed by elastomers, PTFE, gasket, etc.
If shaft sleeve is used, one more static leakage path will be there between shaft and shaft sleeve. This leakage is arrested by O-Ring or gasket.
Although the main closing force on primary seal faces is normally provided by the pressure in the stuffing box, some force is required to keep them closed during startup and shutdown and to take care of the shaft movement. This force is supplied by a single large spring, a series of small springs, or a bellows arrangement.
Types of Mechanical Seal
There are many types of seals each having definite advantage as under.
Inside Seals When a seal is mounted inside the stuffing box of the pump, it is called an inside seal. Inside seals are more difficult to install and maintain. However, main advantage is that it is possible to control the seal environment inside the stuffing box.
Outside Seals An outside seal is located outboard of the pump stuffing box. Where stuffing boxes are shallow and it is not possible to install a seal inside the stuff ing box, it is installed
outside. It is also easy to install and maintain. Due t o lake of heat dissipation from below the seal faces, outside seals are suitable for low t emperature, low speed and low pressure applications.
Balanced Seal All seals are available in either unbalanced or balanced versions. A seal is unbalanced when fluid force to close the seal faces (due to the area of rotating seal face exposed to the pumped fluid in stuffing box) is greater than f orce acting on rotating seal face at the area of contact (pressure gradient between rotating and stat ionary seal faces). In simple terms, it has a seal closing force in excess of the actual pressure to be sealed. In a balance seal as seal face is subject to low force, less heat is generated and seal life is more. As a stepped shaft sleeve is required for balancing, coat of a balanced seal is higher than unbalanced seal.
70
30 Balanced Seal Design
–
To balance a seal, area of rotating seal face exposed to stuffing box pressure is reduced using a stepped shaft sleeve. In a standard 70 – 30 balanced seal design used by most mechanical seal manufacturing companies, only 70 % of rotating seal face area is exposed to stuffing box pressure as shown in above sketch.
Double Seals Double mechanical seal arrangement is used to handle toxic, volatile, hazardous or abrasive fluids. In a double seal arrangement, there are two seals with a fluid circulating between them. The fluid that circulates between the seals is called a barrier fluid if its pressure is higher than stuffing box pressure and it is called a buffer fluid if its pressure is lower than stuffing box pressure. The two seal faces are generally installed in one of the three different configurations as shown under.
Back to back or facing in opposite directions
This configuration requires a higher barrier fluid pressure between t he seals. In this arrangement an inner seal leak will cause a dilution of the product. In case of failure of the barrier fluid system, the inner seal can blow open dumping the pump contents to the environment.
Tandem or facing in the same direction
In this configuration two glands are required to house both seals and this adds to the cost as well as the axial space requirement. A low pressure buffer fluid is circulated between the seals, eliminating the possibility of product dilution. In this arrangement loss of buffer fluid will not cause the seal faces to open. This configuration is generally found in Oil Refinery applications.
Face to face or facing towards each other
Face to face configuration is a compromise between the "back to back" and the tandem arrangements. Here half the seal is housed in the stuffing box and the other half outside it. In this arrangement a lower pressure buffer fluid is supplied between seal faces.
Catridge Seals The catridge design changes none of the functional components of the basic seal (conventional seal). In a catridge seal, all items are containerized and preset t o working dimensions. They eliminates need to scribe lines and make critical measurements during seal assembly. Such seal installation requires only tightening of the gland bolts.
Methods of Environment Control
The successful and reliable operation of a mechanical seal is dependant upon the conditions that are imposed on the seal assembly during running. The fluid being sealed fills the stuffing box in which the seal is mounted and thus the physical and chemical nature of this liquid will have direct effect on seal operation and life. S lurries and fluids carrying solid particles are especially dangerous as there is a tendency for solid particles to collect in the vicinity of the mating faces and finally even entering the fluid film gap between the mating fa ces. Hard particles entering this gap will cause premature seal face failure.
Improved seal operation is possible by controlling the environment surrounding the seal. The most commonly used methods for control are flushing and quenching.
API Gland
Plan # 62
–
Flushing In flushing a fluid is injected (through connection F as shown in API Gland – Plan # 62) into the stuffing box such that it impinges or jets onto the mating faces. This fluid may be the same fluid that is being sealed, tapped from a point at a higher pressure
than that existing in the stuffing box, or any other fluid, preferably at a lower temperature, that may be permitted to mix with the sealed fluid. Flushing effectively aids cooling of the seal mating face area. In addition, the introduction of a pressurized clear fluid ensures that solid particles present in the sealed media do not collect near the sealing faces.
Quenching In quenching a fluid is introduced (through connection Q as shown in API Gland – Plan # 62) on the atmospheric or outer side of the seal mating faces that either helps in cooling or in maintaining a require temperature at the mating faces. This also creates a barrier between the atmosphere and seal faces as the a tmospheric air creates problem to seal faces in some cases. Few such applications are given below.
•
When pumping cool media (say at – 40 deg. C), moisture in the a tmosphere
condenses and ice is formed below seal face hindering its operation. •
In case of high temperature oils when vapors keep on escaping in the
atmosphere, they come in contact with oxygen and burn. These carbon particles cerate problem in seal area. •
Crystallizing media forms crystals when water in it gets evaporated because of the
atmospheric air present blow seal faces. These crystals create problems for seal faces.
The American Petroleum Institute (API) issues guide lines to help petroleum people select and then pipe various t ypes of controls for mechanical sealing applications. These piping arrangements are described in API standard 610.
Equipment Parameters
For satisfactory seal performance, equipment parameters should be as under.
Radial movement of shaft (runout / deflection) shall be less than 0.08 mm. Axial movement of shaft (end play) shall be less t han 0.26 mm. Stuffing box face squareness (face runout) shall be less than 0.05 mm. Stuffing box bore concentricity (with respect to shaft) shall be less than 0.13 mm. Stuffing box shall be free of burrs and sharp edges. Shaft / sleeve shall be free of burrs and sharp edges.
Please check manufacturer’s drawing /instruction for above parameters.
Installation Instructions
A conventional (component) seal is one where each part of the seal must be assembled on the equipment individually. This requires considerable skill and significant time as compared to installation of a catridge seal. During installation of a mechanical seal take care of following.
•
Assemble seal parts in a clean environment.
•
Do not use hammer for assembly as seal faces are delicate and may crack /
break. •
Check that seal parts, gland and stuffing box are free from burrs, sharp edged
and deep scratches / damage. •
Check surface finish at elastomer area to be as per manufacturer’s
recommendation. •
Check that set screws on either the rotary unit or the drive collar of the seal
assembly are free in the threads. •
Confirm hardness of shaft / sleeve to be such that after tightening set screws,
rotating assembly does not get loose (if set screws are tightened against a hard surface, they will fail to hold assembly in desired position during operation). Alternatively, use hardened set screws. •
It is a good practice to check fitting of shaft sleeve, rotary assembly and gland
without O-Rings to ensure that are fitting freely before assembling them with O-R ings. •
Use correct size O-Rings at all places.
•
Do not use used O-Rings and gaskets.
•
Never use "glued together" O-Ring for any "dynamic" application. A hard spot will
be created that will interfere with the movement of the O-Ring. •
Lubricate shaft and secondary seal (O-Rings / bellows) as per manufacturer’s
recommendation. If assembling is difficult, apply water or soap water as lubrication on shaft / sleeve. Rubber bellow seals should be lu bricated with Vaseline. Don’t apply silicon grease on them. EPR (Ethylene Propylene Rubber) elastomers should not be lubricated with petroleum based oil. For EPR material use silicon grease. •
Check that all O-Rings are protruding out of respective groove provided on mating
part. •
Install seal at its correct operating length as per manufacturer’s drawing.
•
After assembly of rotating head, check for free movement of seal ring by
compressing and releasing the rotary head. •
Check direction of helix of coil for single spring seal. Helix should be R.H. for C.W.
rotation and L.H. for C.C.W. rotation when looking at seal face. •
Gland bolts or nuts should be tightened only enough to effect a gasket seal at the
stuffing box face. This can be achieved by initial finger tightening and further tightening with ½ to ¾ turns. Over tightening could result in distortion of seal faces. •
Cartridge type seal assemblies are provided with axial location plates that hold the
assembly together before installation in the equipment. Make sure that the axial location plates are moved out of the grooves provided on the shaft sleeve after their fitting. •
When seal assembly is complete, connect all piping, check that all environmental
controls have been connected, and all unused holes in the stuffing box / gland are plugged.
Start-up Procedure
Take care of following before starting equipment and during its operation for the first time after installation of mechanical seal.
•
Equipment should be aligned with the driver as per manufacturer’s
recommendations. •
Check the shaft for free movement. Manually rotate the shaft several turns. If
shaft binds due to any reason, investigate and correct it. •
Activate all auxiliary systems like flush, quench, barrier lines and vent the stuffing
box until all trapped air has been released. •
Pump should have adequate NPSH (for its running without cavitations).
•
Equipment should run without vibration.
•
No noise should come from stuffing box.
•
Excessive heat generation should not be there. This may be due to stationary
parts contacting the rotating shaft or rotating seal parts contacting the housing of the equipment. •
Examine the seal. Slight leakage should stop when the faces “wear in”.
If dynamic testing of a seal is to be carried out, it should be carried out at maximum stuffing box pressure. The seal should be tested for at least 3 hours. Le akage should be less than specified in purchase order. If no leakage rate is specified in p urchase
order, leakage at the rate of maximum 2 to 3 drops per minute is considered to be acceptable.
Causes of Seal Leakage
The operating life of a seal is complete when e ither face has worn entirely. If either face has completely worn, the cause of failure is evident and no further inspection is required unless this occurred in a very short time. If both faces are intact, seal parts shall be inspected. Major seal problems and possible causes are as under.
Seal Problems
Possible Cause / Corrective Action
Seal spits and sputters
Seal fluid vapourizing at seal interfaces. This can be due to inadequate
(“face popping”) in operation.
seal faces or seal unbalance.
Seal drips steadily.
This can happen if seal faces are not flat, distorted or damaged. Distortion of gland plate due to over tightening. Damage to secondary seal during installation. Over aged O-Rings. Spring failure. Erosion / corrosion of seal parts.
Seal squeals (gives sound) during Inadequate liquid to lubricate seal faces. operation Accumulation of carbon dust
Inadequate liquid to lubricate seal faces.
outside the gland. Short seal life.
Presence of abrasive in the fluid. Misalignment of the equipment with its driver. High vibration.
Note: API 682 (Shaft sealing systems for centrifugal and rotary pumps) requires that the sealing system supplied, “have a high probability of meeting the objective of at least three years of uninterrupted service while complying with emission regulations”.
Mechanical Seal Manufacturers
Internet site addresses of some mechanical seals manufacturers are as under
www.johncrane.co.uk www.burgmannindia.com www.leakproofseals.com
Acknowledgement: Sketch of basic mechanical seal is taken from internet site – w
Mechanical Seal – Technical Information (Part 1) Filed under: General Maintenance — K P Shah @ 9:51 pm
The ability of a mechanical seal to meet its performance objectives depends upon a wide range of factors involving equipment design, operating conditions and selection of the type of seal and the material of construction. As information on equipment design (types of mechanical seals, methods of environment control and equipment parameters) and operating conditions (Start-up procedure) are covered in a blog on Mechanical Seal – Practical Information, information about selection of type of mechanical seal and material of construction is given in this article.
Selection of Type of Mechanical Seal
The design, arrangement and material selection of a seal is basically determined by pressure, temperature, speed of rotation and characteristics of the pumped medium. Shaft diameters of 5 To 500 m m, pressures from 10 torr (vacuum) to 250 bar, temperatures from -200°C to +450°C and sliding velocities up to 150 m/s limit the operating range of mechanical seals. Type of a mechanical seal for various parameters may be selected as under.
Single Seal Type Inside Temperatures
Outside
Unbalanced
Balanced
Unbalanced
Balanced
Up to 120 ºC
120º C – 205
ºC Over 205 ºC
Single Seal Type Inside Pressures Up
to
10
Outside
Unbalanced
Balanced
Unbalanced
Balanced
Kg/Cm2 10
to
35
35
Kg/Cm2 Over Kg/Cm2
Single Seal Type Inside Speeds
Outside
Unbalanced
Balanced
Unbalanced
Balanced
7.6
7.6 to 15.2
Up
to
m/s
m/s Over
15.2
m/s m/s = meters per second
Double Seal Double seal arrangement with additional seal supply systems or buffer fluid systems may be required depending on the quality of the medium (toxic, inflammable, crystallizing, corrosive, abrasives in fluid, etc).
Due to restriction on length of a blog, I have posted this blog in two parts. Please read Part 2 also.
Published on September 09, 2008
Mechanical Seal – Technical Information (Part 2) Filed under: General Maintenance — K P Shah @ 9:56 pm
Material of Construction
Seal components can be divided into three major categories – seal faces, secondary sealing elements and metal components.
Seal Faces
The rotating and stationary sealing faces commonly referred to as primary seal members are the most important components of a mechanical seal. They shall be selected based on their compatibility with the fluid being pumped. Following materials are widely used as seal face material.
Resin impregnated Carbon This is the normal rotary seal face material recommended in most general purpose application involving corrosive fluids. This carbon exhibits good resistance to thermal shock and good dimensional stability over a wide temperature range. It has also low permeability and good thermal conductivity.
Metal impregnated Hard Carbon This is an antimony impregnated hard carbon that is specially suited for extreme heavy duty application involving non-corrosive media. Boiler feed wa ter and hydrocarbon service seals with hard carbon as a mating face have a much longer service life. Hard carbon exhibits better abrasive resistance and emergency dry running characteristics.
Ceramic This is a super fine grain high Alumina ceramic material (99.5 % Al2O3) that exhibits excellent low wear characteristics. It is the best seal face material for highly corrosive chemical services. 95.0 % purity material may be used for light duty application.
Tungsten Carbide This is universally accepted hard seal face material. It is available in two f orms –
nickel bonded and cobalt bonded. Solid seal rings are offered as a standard as against shrunk-fit faces with their inherent limitations.
Silicon Carbide Technologically this is the best seal face material available to date. It is available in two varieties, reaction bonded and sintered. It is highly resistant to thermal stress and corrosion in high temperature oxidizing atmospheres. It has low wear properties and is an idle seal face material for most of sealing applications. Silicon carbide also exhibits better dry run capabilities making it an ideal choice for critical duties in the nuclear and thermal power industries.
Glass filled PTFE It is offered as a standard seal f ace material on outside mounted PTFE bellows type seals. It is recommended for corrosive applications. Safe working temperature rang for PTFE is -200 to 260° C.
Other Materials Alternate face materials are available for custom seals and other special applications. Seal faces of stainless steel with stelliting and Ni-resist are available. Cast iron faces are also available for certain non-critical applications.
Note: Carbon face is made in many grades and is priced from the cheap / mass-produced grades to expensive metal-powder impregnated varieties. While ordering spare carbon ring from local supplier, specify correct grade of carbon for your application.
Properties of various face materials are as under
Material
Density
Thermal
Hardness
Max. Temp.
Conductivity gram/cm3
Limit, ºC W/mºC
Carbon,
resin 1.83
impregnated
6
100 BHN
275
Carbon,
2.15
8
115 BHN
350
15
100
1500
400
antimony impregnated Tungsten Carbide
Solid
Vickers
(6% Co) Silicon Carbide
3.1
145
2400
1650
Vickers Alumina
Oxide 3.9
35
1800
(99.5 %)
175
Vickers
Seal Pressure – Velocity Limitations
Seal faces require cooling and lubrication to function properly. The hydraulic pressure acting on the seal faces and the rotating speed of the rotary seal will generate heat. This heat limits seal design and material. The PV – (face pressure x velocity) capability of two opposing material is indicative of an ability to sustain a fluid film for long operational life. Typical PV – Limits of face material combinations in nonlubricating fluids, i.e. watery substances are as under.
Primary (rotating) Ring
Mating (stationary)
PV Limit (bar x m/s)
Ring Glass-Filled PTFE
Ceramic
/
Silicon 61.3
Carbide Carbon
Cast Iron
245.2
Carbon
Ceramic
245.2
Carbon
Tungsten Carbide
1225.9
Carbon
Silicon Carbide
1471.1
Tungsten Carbide
Tungsten Carbide
249.2
Silicon Carbide
Silicon Carbide
858.1
Note: For lubricating fluids multiply number by 1.5
Seal face surface finish and Seal face flatness
To maintain a healthy lubricating fluid film between seal faces they are lapped to make them flat and smooth. If faces are not flat, waviness will generate hydrodynamic lifting force on seal faces as we try to compress non-compressible liquid trapped between the lapped faces. Seal surfaces shall be smooth also to reduce friction between them by increasing contact area.
There is often confusion between the terms "Seal face flatness" and "Sea l face surface finish". Seal face surface finish addresses the subject of roughness, and is measured in terms of "rms" (root mean square) or CLA (center line average). One of the ways to measure roughness is by comparing our sample to standards that have been polished to different degrees of roughness. Flatness is a different term that describes a level surface that has no elevations or depressions. We use term waviness to describe this condition when we refer to mechanical seal faces. It is this flatness that is of the most concern. One can read the flatness by using an optical flat and a monochromatic light source as explained below.
Flatness is measured by using light characteristic – that when two lights of the same wave length interfere with each other, the light disappears and the ref lecting piece goes black. A monochromatic or single wave length light source (mono means one, and chromatic means color).) is used for this. Most companies use a pink color that comes off a helium gas light source. This color has a wave length of just about 0.6 microns (0.000023 inches). To measure flatness, an opt ical flat (a precision ground and polished clear glass of optical quality) is placed on the piece to be measured. T he monochromatic light is aimed at the piece and this light reflects off of the piece back through the optical flat causing interference light bands. If the distance between the optical flat and the piece we are measuring is one half t he wave length of helium, or an even multiple of the number, the band will show black. This is referred to as a helium light band and because it is one half the wave length of helium it measures 0.3 microns or 0.0000116 inches. Flatness is checked by comparing the pattern we see with a chart supplied by the measuring equipment manufacturer.
Surface roughness and flatness inspection
Flatness of lapped faces should be within following light bands. Carbon and GFT: 2 to 3 light bands. TC, SiC and Ceramic: 1 to 2 light bands. For high pressure application (> 40 bar), faces should be lapped within 1 light band.
Carbon graphite faces relax after lapping. Although lapped to less than one light band by the seal manufacturer, you will see readings as high as three light bands if you check the faces. These faces should return to flat once they are p laced against a hard face that is flat.
Seals that are going to be used in cryogenic (cold) service should be lapped at the cryogenic temperature.
Finished faces shall have following average surface finish. Tungsten Carbide: 0.01 µm Silicon Carbide: 0.04 µm Hard Carbon: 0.1 µm Ceramic: 0.07 µm
Hydrodynamic Grooves. Sometimes hydrodynamic grooves are provided on hard face as shown below for
effective lubrication between faces.
Hard Seal Face with Hydrodynamic Grooves
Secondary Sealing Elements
Secondary seals perform the function of sealing between mechanical seal elements as well as sealing the mechanical seal and the equipment. They are either static or dynamic type in the form of O-rings, wedges, bellows and gaskets.
For information on gaskets used for sealing seal and the equipment, please refer blog on gaskets. Bellows and wedges are made from PTFE. Bellows are also made from elastomers and metal. O-rings are made from elastomers.
Elastomers
To be classified as a true elastomer you should be able to compress an O-ring and have it return to 90% of its original shape in less than five seconds after th e compression force is removed. It is this elasticity that gives the compound its memory and eliminates the need for external loading to seal. I f the compound does not return to 90% of its original shape in five seconds or less it is called a "plastic" material and becomes less desirable as a dynamic seal in mechanical seal design. Most of Perfluoroelastomers are plastics. Generally one of the following elastomer materials is used to make an O-ring.
•
Butyl
•
Buna N
•
Neoprene
•
Ethylene propylene
•
Fluorocarbons: They are sold by manufacturers under their style / produce
number. Dupont E60 Viton ®, 3M Fluorel 2174, Parker 747-75 and Parker V 884-85 are typical examples. •
Perfluoroelastomers: Chemraz (a registered trademark of Greene, Tweed & Co.)
or Kalrez ® (a registered trademark of Dupont, USA) are typical examples. They are used for high temperature and aggressive chemical applications. Their chemical resistance is often compared with PTFE. They are very expensive compounds.
The O-ring selected must be chemically compatible with fluid to be handled. It is very common to clean and flush process lines with a solvent or steam. The O-ring selected must be chemically compatible with them also. Most of the chemicals can be handled by either fluorocarbon (Viton/ Fluorel) or Ethylene P ropylene. Ethylene Propylene is easily attacked by any petroleum product so be careful with the type of lubricant you use to lubricate it. For all practical purposes silicone grease is probably the safest lubricant but to be sure check for its compatibility.
Each of these elastomers has an upper and lower temperature limit. Although the elastomer may be chemically compatible with the sealing fluid it could still fail if the temperature limit is exceeded. Safe temperature range for various elastomers is as under.
Elastomer
Temperature range °C
Butyl
-40 to 130
Buna N (Nitrile)
-40 to 105
Neoprene
-40 to 120
Ethylene propylene
-40 to 150
Flurocarbon (Viton ®)
-20 to 200
Chemraz
-30 to 205
Kalrez ® (many grades are available)
-20 to (218 to 315 based on type of grade).
Note: Elastomers are poor conductors of heat. Cooling one side of the O-ring does not always allow the coolant to conduct to the hot side.
Most of the O-ring compounds are available in a wide range of durometer or hardness. The average mechanical seal uses a durometer of 75 to 80 ( as measured on the shore A scale), but harder durometers are available for high pressure applications.
One measures O-ring sizes by the inside diameter (D) and the cross section diameter (d). O-rings are the most precision rubber part that one can purchase. They are manufactured to a tolerance of ± 0.08 mm.
The maximum volume of the O-ring should never be more than the minimum volume of the gland groove. The groove depth must be less than t he O-ring cross-section and the groove width must be larger than the O-ring cross-section.
Identification of O-ring material by Burning Test (destructive test). To identify Viton, Burn Test may be carried out. When ignited, Neoprene and Ethylene propylene burns with a flame where as Viton does not burn with a f lame.
Metal Components
Metal is used for making mechanical seal hardware. This hardware, depending on seal design can include sleeves, retaining rings, set screws, pins, springs, bellows and glands. Although mechanical seal have some unique requirements, the material selection generally does not differ much from material selection for the equipment. As seal components are thinner than equipment components, ma terials offering best corrosion resistance are selected for hardware. Many of the common names used for material designation are actually trade marks of the material manufacturer. Following material are widely used for making mechanical seal hardware.
Stainless Steel 316 AISI 316 (UNS S31600) is considered the base material for most sea l designs. It should not be used in service with high chlorides since it is susceptible to p itting corrosion.
Alloy C-276 Alloy C-276 (UNS N10276) is one of the most widely used high alloy material used for aggressive environments. It is used for all major seal components including sleeves, glands and fasteners. C-276 is a nickel-molybedenum-chromium alloy. It is used as
standard alloy for springs and is defined as the default spring material in API 682 (2004).
Alloy 20 Alloy 20 (UNS N08020) is a nickel-chromium-molybdenum alloy. It was originally developed for hot sulfuric acid application. It is used for applications that cause stress corrosion cracking.
Alloy 400 Alloy 400 (UNS N04400) is a copper-nickel alloy that exhibit good corrosion resistance against many chemicals. It is used for sea water, sulfuric acid, hydrochloric acid, hydrofluoric acid and alkalies.
Alloy K 500 Monel alloy K 500 (UNS N05500) is used for components requiring high strength like set screws and fasteners.
Alloy 350 Alloy 350 (UNS S35000) is a chromium-nickel-molybdenum alloy that exhibit high strength in high temperature applications. It is mainly used for making bellows.
Alloy 718 Alloy 718 (UNS N07718).is a nickel-chromium alloy that exhibits excellent corrosion resistance and high temperature properties. The material is mainly used for making welded metal bellows. This alloy has been adopted as the default mater ial for Type C seals in API 682 (2004).
Note: UNS stands for Unified numbering system.
For more information on mechanical seal material selection please refer API Standard 682, 2004 –“Pumps-Shaft Sealing Systems for Centrifugal and Rotary P umps”.
Acknowledgment: Information about metal components in this article is briefly reproduced from a paper on Material Selection for Mechanical Seals by Michael Huebner in Proceedings of the Twenty Second International Pump Users Symposium 2005.
Published on September 09, 2008
